1. Field of the Invention
This invention relates to an optical information reproducing apparatus which optically reads an information from a recording medium, and more particularly to its tracking control.
2. Description of the Prior Art
There is a well known method to carry out tracking control required for an optical information reproducing apparatus, in which two-divided parts of a photodetector differentially detects, an unbalanced quantity distribution of reflected light from an information track to fetch signals corresponding to a tracking error. Such a tracking error detecting method is disclosed, for example, in U.S. Pat. No. 4,059,841. The method, however, has a problem that the tracking control tends to be unstable due to a variation of offset of the tracking error signals caused by an inclination of the recording medium or a displacement of an image on the photodetector.
There is also a well known method which divides a photodetector into four quadrants and detects a tracking error by a phase difference between a signal corresponding to a sum of quantities of lights incident on the first and third quadrants and a signal corresponding to a sum of quantities of lights incident on the second and fourth quadrants. (This method will be hereinafter called "phase difference tracking method".) This phase difference tracking method is disclosed, for example, in Japanese Pat. Appln. of laid-open No. 52-93222. The method can substantially solve the problem previously mentioned. However, it has still a problem that, when reading an information from a recording medium in which the information is recorded as irregularities on a surface with a phase depth less than a quarter wave length, a displacement of an image on the photodetector causes a variation of offset of the tracking error signal, resulting in a tendency to unstable tracking control. We inventors discovered that the problem was due to the following reason.
When the far field image of a light spot formed on a photodetector is divided into four quadrants by a first straight line substantially parallel to a direction in which an information track is extended and a second straight line substantially perpendicular to the direction, there is a phase difference between quantity changes of light incident on the first and second quadrants located on one side to the second line and of light incident on the third and fourth quadrants located on the other side. Since the phase difference occurs with respect to the tangential direction of the information track in the far field image, it will be hereinafter called "tangential phase difference". When an information is recorded in a recording medium as irregularities of a surface with a phase depth of an integral number times a quarter wave length, the tangential phase difference becomes zero when focussing. But a defocussing causes a tangential phase difference corresponding to a defocussed quantity. On the other hand, when the pit phase depth deviates from an integral number times a quarter wave length, the tangential phase difference does not become zero even when accurately focussing. This causes an offset in the tangential phase difference. The offset in the tangential phase difference varies depending on the deviation of the pit phase depth. When the far field image is symmetrically formed with respect to the first and second straight lines, the detection of the phase difference between the sum of outputs from the first and third quadrants and the sum of outputs from the second and fourth quadrants causes the tangential phase difference to be canceled and not to become the offset of the tracking error signal.
However, if the far field image formed on the photodetector is asymmetrically displaced with respect to the first straight line, the tangential phase difference is not canceled to become the offset of the tracking error signal. Especially, when the pit phase depth is, less than a quarter wave length, the offset of the tracking error signal varies in a direction to increase the tracking error. A bias of an objective lens in the radial direction causes the offset of the tracking error signal develop to cause a positive feedback so as to further expand the bias. Thus, if the tracking control is released, the tracking control system runs away and the carriage transferring the optical head also runs away.